Method for welding a case hardened component

ABSTRACT

A method for producing a welded part from two components, where at least one of the components has a hardened surface. The method can include case hardening the surface of one of the components using a salt bath nitriding process and then welding the case hardened first component to the second component by gas metal arc welding (GMAW).

BACKGROUND OF THE INVENTION

Some home appliances, such as washing machines, use processes to hardenthe surface of components for increased wear and/or corrosionresistance. Salt bath nitriding is one example of a process by which acase hardened surface can be created on a component.

Whirlpool has used salt bath nitriding on the spin tube of the gear caseon some washing machines to increase the corrosion resistance of thespin tube, which is exposed to wash liquid during operation of thewashing machine. The spin tube, which is made of cold-rolled steel, iswelded in place before nitriding because the prior art had establishedthat nitrided parts cannot be successfully welded. Welding a nitridedcomponent may lead to an unacceptable level of porosity in the weld.DE3429994 teaches that after salt bath nitriding, surfaces are no longerweldable. The textbook, Laser Welding: a Practical Guide confirms thatnitrided surfaces are generally unsuitable for fusion welding processes,since the weld will remove the surface hardness in the vicinity of theweld.

BRIEF DESCRIPTION OF THE INVENTION

According to one aspect of the invention, a method for producing awelded part comprising a first component welded to a second componentincludes case hardening the surface of the first component by a saltbath nitriding process, and welding the case hardened first component tothe second component by a gas metal arc welding process using analuminum-bronze wire electrode, wherein the weld between the first andsecond components is substantially free of porosity and the surfacehardness of the first component in the vicinity of the weld issubstantially undiminished.

According to another aspect of the invention, an appliance comprises acase hardened first component welded to a second component, wherein thefirst component is case hardened by a salt bath nitriding process priorto welding the case hardened first component to the second component bya gas metal arc welding process using an aluminum-bronze wire electrode.The weld between the first and second components is substantially freeof porosity and the surface hardness of the first component in thevicinity of the weld is substantially undiminished.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a perspective partially cutaway view of an appliance accordingto a first embodiment of the invention;

FIG. 2 is a partial sectional view of the drive portion of the washingmachine of FIG. 1;

FIG. 3 is a side view of a welded assembly of the appliance from FIG. 1;

FIG. 4 is a close-up view of section IV of the welded assembly from FIG.3;

FIG. 5 is a flow chart illustrating a method for producing the weldedassembly from FIG. 3; and

FIG. 6 is a schematic view of a gas metal arc welding process forproducing the welded assembly from FIG. 3.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

According to one aspect of the invention, an appliance can be providedwith at least one welded assembly, where at least one component of thewelded assembly can be subjected to a salt bath nitriding process priorto welding. One example of a welded assembly that can be produced usingthe method of the invention is a spin tube and gear case of a clotheswashing machine. However, the method of the invention can be used toproduce other welded assemblies for appliances in which it would bebeneficial to case harden at least one of the components of the assemblyprior to welding.

FIG. 1 illustrates an exemplary appliance in the form of a vertical axiswashing machine 10 that may comprise an open top cabinet 12 closable bya hinged lid 14. The cabinet 12 may house a stationary, imperforate,open top wash tub 16. A perforated, open top drum 18 may be rotatablymounted inside the wash tub 16 and define a laundry treating chamber 20having a rotatable clothes mover 22 mounted therein, as is generallyknown in the washing machine art. Rotation of the clothes mover 22 andthe drum 18 may be driven by a motor 24 disposed within the cabinet 12below the wash tub 16, operably coupled to a controller 26. Thecontroller 26 may communicate with a control panel 28 through which auser may select a desired automatic wash cycle.

A typical wash cycle may comprise at least a wash step to wash theclothes load with wash liquid, a rinse step to rinse the wash liquidfrom the clothes load with water, and a spin step to extract excessrinse water from the clothes load. During the wash and rinse steps, theclothes mover 22 may oscillate and rotate relative to the drum 18 tomove the clothes load and/or liquid within the treating chamber 20.During the spin step, both the clothes mover 22 and the drum 18 maytypically rotate at high speeds in a selected direction to extractexcess water from the fabric items. Depending on the type of wash cycleselected by a user through the control panel 28, the clothes load may besubjected to combinations and numbers of wash, rinse, and spin steps.

Referring to FIG. 2, the motor 24 is coupled to the clothes mover 22through an agitator shaft 30 having an upper end fixedly mounted to theclothes mover 22 and a lower end mounted to a pulley 32 driven by a belt34 that rotates around a drive wheel 36 affixed to a reversiblyrotatable motor shaft (not shown), as is well known in the washingmachine art.

A generally hollow spin tube 38 according to the illustrated embodimentof the invention surrounds the agitator shaft 30 such that the agitatorshaft 30 can rotate relative to the spin tube 38. The drum 18 is fixedlyattached to the spin tube 38 and is selectively mechanically coupled tothe agitator shaft 30 via a clutch assembly (not shown) in a gear case40. The spin tube 38 can be welded to the top of the gear case 40. Asshown herein, the bottom of the spin tube 38 is welded to a gear casecover 42 to define a welded assembly 44.

FIGS. 3-4 is a side view and close-up view of the welded assembly 44 ofthe appliance from FIG. 2 including the spin tube 38 and gear case cover42, which are joined at a weld joint 46. The spin tube 38 is formed frommetal, preferably steel, and can have a case hardened surface producedby salt bath nitriding, such that nitrogen and carbon are diffused intothe surface of the metal. The gear case cover 42 is also formed frommetal, preferably steel, and can be welded to the spin tube 38 after thesalt bath nitriding process. As such, the gear case cover 42 does nothave a case hardened surface.

FIG. 5 is a flow chart illustrating a method 50 for producing the weldedassembly 44 from FIG. 3. While discussed in terms of the spin tube 38and gear case cover 42, the method can be applied to produce otherwelded assemblies for appliances in which it would be beneficial to caseharden at least one of the components of the assembly prior to welding.The first step 52 in the method is manufacturing the spin tube 38 andgear case cover 42 for the assembly 44. Processes for manufacturing spintubes and gear case covers are well-known in the art, and will not bedescribed in detail herein. Briefly, the spin tube 38 and gear casecover 42 can be machined with features for attachment to the drum 18 andgear case 40; for example, the spin tube 38 and gear case cover 42 canbe stamped and finished by grinding. Next, the spin tube 38 undergoes asalt bath nitriding process at step 54. Briefly, the spin tube 38 isdipped into a salt bath tank containing a nitrogen donating medium andnitrogen diffuses into the surface of the metal spin tube 38 to caseharden the surface. At step 56, the case hardened spin tube 38 is weldedto the gear case cover 42 using gas metal arc welding (GMAW). Inpreparation for the welding step 56, the spin tube 38 and/or gear casecover 42 may be cleaned mechanically or chemically. The spin tube 38 andgear case cover 42 are joined at weld joint 46, which is substantiallyfree of weld porosity, having extremely low, often near zero, porosity,and the surface hardness of the spin tube 38 in the vicinity of the weldjoint 46 is substantially undiminished from its level after the saltbath nitriding step 54. Finally, the welded assembly 44 can be installedon the washing machine 10 at step 58.

FIG. 6 is a schematic view of a GMAW process 60 for producing the weldedassembly from FIG. 3. After salt bath nitriding, the spin tube 38 ispartially inserted into the gear case cover 42 and a welding gun 62 isused to create an arc between a solid wire electrode 64 and the assembly44. The solid wire electrode 64 melts to create a weld pool and becomesthe filler material for the weld joint. A pulsed-spray transfer mode canbe used to deposit the filler material in a spray of small droplets. Asthe weld pool reaches the proper width, the assembly 44 is rotatedrelative to the stationary welding gun 62 until the end of the weld isreached. More than one pass around the assembly 44 may be required; forexample, a slight overlap of the weld may be required, such as anoverlap of approximately 10°. The centerline of the assembly 44 may beoriented along an approximately horizontal axis, with the welding gun 62fixed at a distance d of approximately 2 mm from the top of the gearcase cover 42 and held an angle α of approximately 15° from normal (i.e.vertical) to the outer surface of the spin tube 38. During welding, ashielding gas flows through the welding gun 62 to protect the weldingarea from atmospheric gases. One example of a suitable shielding gas isargon.

The solid wire electrode 64 used as filler material is analuminum-bronze alloy, the bronze itself being an alloy of copper andtin. The aluminum-bronze alloy is also corrosion resistant. One specificexample of a suitable aluminum-bronze alloy is Luvaweld® SG-CuAl8,available from Luvata, which has a diameter of 1.0 mm.

While many of the variables of the welding process 60 can be modified tosuit the particular components to be welded, one specific set ofvariables for the welding process 60 is set forth in the table below.

Gear Case Cover 0.9 mm Galvanized Steel Spin Tube 2.4 mm Nitrided SteelElectrode Wire 1.0 mm Luvaweld ® SG-CuA18 (Aluminum Bronze) ElectrodeExtension (mm) 15 Shielding Gas 100% Argon Shielding Gas Flow Rate 35(CFH) Programmed Current (A) 125 Arc Tune 40 Actual Current (A) 176Voltage (V) 17 Travel Speed (ipm) 60 RPM 15.0 Approx. Weld Time(seconds) 4.3 Approx. Wire Consumption 20″ measured during welding

There are several advantages of the present disclosure arising from thevarious features of the apparatuses described herein. For example, theembodiment of the invention described above allows for a robust weldbetween two components, where at least one of the components has alreadybeen nitrided. Nitrided spin tubes have previously been used on somewashing machines, but these spin tubes had to be welded in place beforenitriding, since conventional thinking in the art is that nitrided partscannot be successfully welded. Welding before nitriding is expensivebecause the entire welded assembly has to be dipped into the salt bathtank, even though only a portion of the welded assembly requires a casehardened surface.

Previous attempts to weld nitride surfaces were unsuccessful. After saltbath nitriding, a hardened surface of black oxide is created. This blackoxide surface makes welding a problem. The black oxide surface causescontamination in the weld joint and the formation of unacceptableporosity in the weld.

The present invention allows nitrided components to be welded using aGMAW process. The combination of high speed GMAW and the aluminum-bronzefiller material makes it possible to weld on the black oxide surfacecreated by salt bath nitriding. The resulting weld joint is robust andhas extremely low, often near zero, porosity. This process allowsnitriding to be completed before welding, thereby limiting nitriding toonly those components that require it. Since the parts are smaller,larger batches can be nitrided at a time, further reducing costs andmanufacturing time.

While the invention has been specifically described in connection withcertain specific embodiments thereof, it is to be understood that thisis by way of illustration and not of limitation. Reasonable variationand modification are possible within the scope of the forgoingdisclosure and drawings without departing from the spirit of theinvention which is defined in the appended claims.

What is claimed is:
 1. An appliance comprising: a case hardened firstcomponent welded to a second component; wherein the first component iscase hardened by a salt bath nitriding process prior to welding the casehardened first component to the second component by a gas metal arcwelding process using an aluminum-bronze wire electrode; and wherein theweld between the first and second components is substantially free ofporosity and the surface hardness of the first component in the vicinityof the weld is substantially undiminished.
 2. The appliance of claim 1,wherein the gas metal arc welding process comprises using a shieldinggas comprising argon.
 3. The appliance of claim 1, wherein the firstcomponent comprises steel.
 4. The appliance of claim 3, wherein thesecond component comprises steel.
 5. The appliance of claim 1, andfurther comprising at least one of grinding and stamping the firstcomponent prior to case hardening the surface of the first component. 6.The appliance of claim 1, wherein the first component comprises a spintube and the second component comprises a gear case cover.
 7. Theappliance of claim 1, wherein the appliance comprises a clothes washingmachine.
 8. An appliance comprising: a case hardened first componentwelded to an unnitrided second component; wherein the first component iscase hardened by a salt bath nitriding process prior to welding the casehardened first component to the unnitrided second component by a gasmetal arc welding process using an aluminum-bronze wire electrode; andwherein the weld between the case hardened first component and theunnitrided second component is substantially free of porosity and thesurface hardness of the first component in the vicinity of the weld issubstantially undiminished.